Interference spectrophotometers such as Fourier transform infrared (FT IR) spectrophotometers are used to make qualitative and quantitative analyses of various substances including organic and inorganic substances as well as high-molecular materials and semiconductor materials.
An interference spectrophotometer has a main interferometer for investigating samples and a control interferometer for starting collection of data from the main interferometer and for stabilizing the speed at which the movable mirror is slid. The control interferometer provides quadrature control to detect the position of the movable mirror. For this purpose, a phase plate such as a .lambda./8 plate is installed between the beam splitter and the fixed mirror. Interference signals meeting at the beam splitter are separated into P waves and S waves by a polarizing beam splitter. The polarized components are detected by their respective detectors, and the position of the movable mirror is determined from the phase relation between the output signals from the detectors and from the wave number.
When measurements are made with an interference spectrophotometer, it is necessary to move the movable mirror at a constant speed in one direction. If external disturbance such as vibration is imparted to the instrument, the sliding movement of the movable mirror is disturbed. In an interference spectrophotometer, the movable mirror is reciprocated plural times to improve the signal-to-noise ratio of data obtained by measurements. Data obtained at the same position of the movable mirror is accumulated. This procedure is called coherent addition. If the sliding movement of the movable mirror is disturbed by external disturbance, then the coherent addition is destroyed in accumulating data. As a result, the accumulated data will be inaccurate. The technique for detecting the position of the movable mirror by providing quadrature control is essential for coherent addition.